Source: ref [3]
a), b) The internal voltage (quasi-Fermi level splitting) of perovskite films with various transport layers. c) The solar cell as a bucket with holes representing the recombination losses.

Non-Radiative Recombination

Non-radiative recombination is one of the solar cells biggest enemies. Fortunately, unless radiative recombination, non-radiative recombination can be avoided and is therefore a key objective of our research group. Non-radiative recombination originates from traps and defect states in the bandgap allowing charges to relax to the ground state through the interaction with phonons and without the emission of photons. Unfortunately, the location of these defects and the dominant non-radiative recombination pathway is often unknown, which complicates a systematic optimization of the devices.

To quantify, and understand the complex recombination processes and mechanism in the multi-layered perovskite cells, the Perowskite Group applies various optical, and electrical measurement techniques in steady-state and in transient mode (Find out more).

Above, a solar cell is illustrated as a bucket that is constantly filled with water (which represents the generation current from the sun). The water level represents the open-circuit voltage (Voc) and the holes the various non-radiative recombination currents (i.e. recombining charges per time) that limit the cell. Note, under open-circuit conditions, the outflow from the bucket is always equal to the influx. However, the water level (i.e. the Voc) can be maximized by closing all holes for non-radiative recombination losses, only keeping the hole for radiative recombination. Our research on perovskite solar cells shows us that most charges recombine in the interface regions between the perovskite and the transport layers. The associated interfacial recombination current (Jrec) limits the achievable open-circuit voltage in the cells by ∆Voc=kT/q*ln(1/Jrec).

 

References 

  1. Wolff, C. M. et al. Nonradiative Recombination in Perovskite Solar Cells: The Role of Interfaces. Adv. Mater. 31, 1902762 (2019).
  2. Caprioglio, P. et al. On the Relation between the Open‐Circuit Voltage and Quasi‐Fermi Level Splitting in Efficient Perovskite Solar Cells. Adv. Energy Mater.9, 1901631 (2019).
  3. Stolterfoht, M. et al. The impact of energy alignment and interfacial recombination on the internal and external open-circuit voltage of perovskite solar cells. Energy Environ. Sci.12, 2778–2788 (2019).
  4. Wolff, C. M. et al. Reduced Interface-Mediated Recombination for High Open-Circuit Voltages in CH3NH3PbI3 Solar Cells. Adv. Mater.29, 1700159 (2017).
  5. Stolterfoht, M. et al. Voltage-Dependent Photoluminescence and How It Correlates with the Fill Factor and Open-Circuit Voltage in Perovskite Solar Cells. ACS Energy Lett.4, 2887–2892 (2019)..
  6. Stolterfoht, M. et al. Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells. Nat. Energy3, 847–854 (2018).
  7. Stolterfoht, M. et al. How To Quantify the Efficiency Potential of Neat Perovskite Films: Perovskite Semiconductors with an Implied Efficiency Exceeding 28%. Adv. Mater.32, 2000080 (2020).
  8. Zhang, S. et al. The Role of Bulk and Interface Recombination in High‐Efficiency Low‐Dimensional Perovskite Solar Cells. Adv. Mater. 1901090 (2019). doi:10.1002/adma.201901090